New Families of Neutrino

The muon-neutrino : νμ

The neutrino (or more exactly the anti-neutrino) coming out of a nuclear reactor is an electron-neutrino because, in the beta decay process, it is emitted together with an electron. But another weak interaction process, the decay of a charged pion into a muon and a neutrino,was observed at the end of the 40’s. Is the neutrino coming from the pion decay (called muon-neutrino) the same as the neutrino observed in beta decay (called electron neutrino)? Or is it only a theoretical arbitrary and convenient assertion? This important question was made more pressing by the non-observation of the decay of a muon into an electron and a gamma (μ→ e γ). This non observation strongly suggested that electron-neutrino and muon-neutrino were different.

Melvin Schwartz in front of spark chamber at BNL

The answer came at the end of 1959. In the cafeteria of Columbia University (New-York), T.D. Lee was leading a discussion on the possibilities for investigating weak interactions at high energies. In the evening, Melvin Schwartz had the idea that neutrinos from pion decays could be produced in sufficient numbers by particle accelerators and made the first calculations [Sch60]. Lee was immediately enthusiastic. Schwartz contacted Leon Lederman and Jack Steinberger. Then, together with Jean-Marc Gaillard, they proposed a detector to be installed close to the accelerator in construction in Brookhaven: 15 GeV protons colliding on a beryllium target (1011 each second) would produce an abundance of pions decaying in flight into muons and neutrinos : the muons are stopped by heavy material (steel, concrete and lead) while the neutrinos can interact in the detector. The ideal detector was found while looking at the spark chamber built by Jim Cronin and his team, at Princeton.

At the same time, behind the Iron Curtain, Bruno Pontecorvo came up with the same ideas as Schwartz and wrote an experiment’s proposal [Pon59b]. Unfortunately the Soviet scientific authorities did not consider it in their priorities.

The spark chambers (10 tons full of neon gas) were ready at the beginning of 1962. Only few weeks were necessary to observe about 30 interactions in the detector, where output particles were muons,which were identified as due to muon-neutrino interactions [Dan62]. This discovery of a second type of neutrinos was awarded a Nobel prize in 1988.

In the early 60’s, the first neutrino beams were built at the CERN PS accelerator [Ber60]. Two types of detectors were used, spark chambers and heavy liquid bubble chambers, giving the first limits on the mass of the intermediate boson (today known as W) [Blo64,Bie64,Ber64] and providing the opportunity to better study the weak interaction.

The tau-neutrino : ντ

In 1970, Glashow, Illiopoulos and Maiani made the hypothesis of the existence of a second quark family, introducing the “charmed” quark, associated to the strange quark. In November 1974, their hypothesis was confirmed by the observation of the J/Ψ particle at Brookhaven and SLAC simultaneously. Second family of neutrinos, second family of quarks: a nice bridge begins to be drawn between leptons and quarks.

After the discovery of the muon-neutrino and the second family of particles, an immediate question raised: are there more families? A first answer came in 1975 with the discovery of the tau lepton by Martin Perl and his team in the SLAC electron-positron storage ring [Per75]. It was the third lepton family. A few months later the b quark was discovered at SLAC; it was the third quark family. It took about 25 years to observe directly the tau-neutrino associated to the tau lepton. This was done at Fermilab in DONUT, an emulsion experiment which observed the charged current interactions of the tau-neutrino by identifying the tau lepton as the only lepton created at the interaction vertex [Kod01]. Science historians may still discuss if the birth of the tau-neutrino was in 1975 or 2001!